1. Field of the Invention
This invention involves improvements to communications systems and methods in a wireless discrete multitone spread spectrum communications system.
2. Description of Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. They must make highly efficient use of the scarce bandwidth resource to provide good service to a large population of users. Code Division Multiple Access (CDMA) protocol has been used by wireless communications systems to efficiently make use of limited bandwidths. The protocol uses a unique code to distinguish each user""s data signal from other users"" data signals. Knowledge of the unique code with which any specific information is transmitted, permits the separation and reconstruction of each user""s message at the receiving end of the communication channel.
Adaptive beamforming technology has become a promising technology for wireless service providers to offer large coverage, high capacity, and high quality service. Based on this technology, a wireless communication system can improve its coverage capability, system capacity, and performance significantly.
The personal wireless access network (PWAN) system described in the referenced Alamouti, Stolarz, et al. patent applications, uses adaptive beamforming combined with a form of the CDMA protocol known as discrete multitone spread spectrum (DMT-SS) to provide efficient communications between a base station and a plurality of remote units. Every effort must be made to avoid loading normal, high priority traffic channels with system management information that has a lower priority. An example of system management information is the characterization of channel quality factors that are not immediately needed to control the realtime operation of the network. What is needed is a way to offload the communication of system management information from high priority traffic channels.
The invention disclosed herein is a new method to make the most efficient use of the scarce spectral bandwidth in a wireless discrete multitone spread spectrum communications system. Each remote station and each base station in the network prepares an error detection code, such as a cyclic redundancy code (CRC), on each block of data to be transmitted over the traffic channels. The CRC value computed for a given block of data is a unique mapping of the data block that characterizes the data block. Any change in the data block will result in a different CRC value. Each data block to be sent is numbered with a block number to distinguish it from other data blocks being sent by a sending station. A data message is formed by concatenating the data block with the block number. A CRC value is computed for each data block. An error detection message is formed by concatenating the CRC value with the block number. The sending station prepares the data message by forming data vectors that will be spread using the discrete multitone spread spectrum (DMT-SS) protocol to distribute the data message over a plurality of discrete tone frequencies, forming a spread signal for the traffic channel. In accordance with the invention, the sending station prepares the error detection message for transmission over the link control channel of the network. The sending station prepares the error detection message by forming a link control channel vector that will be spread using the discrete multitone spread spectrum (DMT-SS) protocol to distribute the data message over a plurality of discrete tone frequencies, forming a spread signal for the link control channel. A link control channel is associated with communications session using the traffic channels. Normally, the link control channel carries control information needed by the sending and receiving stations during a session using the traffic channels. However, its capacity is under-utilized. In accordance with the invention, the instant of transmission of the error detection message is allowed to be different from the instant of transmission of the data message. This permits the error detection messages to be transmitted when capacity is available on the link control channel. The receiving station buffers the error detection messages it receives from the link control channel, so that they are accessible by their block numbers. When the receiving station receives a data message on the traffic channel, it performs a CRC calculation on the data block in the message to obtain a resulting new CRC value. The new CRC value is also buffered at the receiving station with the block number so that it is accessible by its block number. Then, when both the received error message and the new CRC value are both available at the receiving station, they are matched by their common block number. The received CRC value in the error detection message is compared with the new CRC computed from the received data block. If the comparison determines that there is a difference in the values, then an error signal is generated. The error signal can be processed and used in several ways. The error signal can initiate a negative acknowledgement signal to be send from the receiving station back to the sender requesting the sender to repeat the data block transmission. The error signal can initiate an update in the spreading and despreading weights at the receiving station in an effort to improve the signal and interference to noise ratio of the traffic channel. The error signal can initiate an alarm to be used for other realtime control. Or, the error signal can be logged for the compilation of a longer term report of the traffic channel quality.
Currently, the invention has advantageous applications in the field of wireless communications, such as cellular communications or personal communications, where bandwidth is scarce compared to the number of the users and their needs. Such applications may be effected in mobile, fixed, or minimally mobile systems. However, the invention may be advantageously applied to other, non-wireless, communications systems as well.